In recent years, electronic devices that can be operated by touching a screen with a finger, a pen, etc., have proliferated. In addition, for a method of detecting a touch position on a display screen, a method is known in which a plurality of optical sensors are provided in a display panel and a shadow image which is created when a finger or the like approaches the screen is detected using the optical sensors. In the method of detecting a shadow image, when the illumination of outside light is low (the surroundings are dark), it becomes difficult to distinguish between a shadow image and a background in an image obtained by the optical sensors and accordingly a touch position may not be able to be detected properly. In view of this, for display devices including a backlight, a method is also known in which a reflection image which is created when backlight light hits a finger is detected using optical sensors.
A display device having a plurality of optical sensors provided in a display panel is described in, for example, Patent Document 1. In addition, Patent Document 2 describes a liquid crystal panel including, as shown in FIG. 21, a pixel portion PP and an infrared detecting portion ISP. The pixel portion PP is provided with a first TFT (T1), a transparent electrode TE, a reflecting electrode RE, etc., and the infrared detecting portion ISP is provided with a capacitor C, a second TFT (T2), etc. The reflecting electrode RE is provided with a transmission window W1 for allowing the transparent electrode TE to be exposed therefrom and an opening window W2 for allowing a pyroelectric thin film PE1 in the capacitor C to be exposed therefrom. The opening window W2 is provided to facilitate the application of infrared radiation which is intentionally provided by a user from outside the liquid crystal panel, to the pyroelectric thin film PE1. In addition, Patent Document 3 describes the provision of a backlight including a non-visible light source that emits non-visible light which can be received by optical sensors, in a liquid crystal display device having optical sensors.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2007-102154    [Patent Document 2] Japanese Laid-Open Patent Publication No. 2004-264846    [Patent Document 3] Japanese Laid-Open Patent Publication No. 2008-3296